Distributors are used to distribute a liquid throughout an area from a liquid feed source. Specifically, in an absorption tower a liquid is distributed across the top of a packed bed within the tower. A gas flows through the tower in generally counter-current flow to the liquid but it can also flow co-currently. The liquid is used to absorb a chemical out of the gas or a gas is used to strip a volatile component from a liquid. Examples in sulphuric acid production include absorption of sulphur trioxide gas, SO3, or of water vapour into a strong sulphuric acid solution; also the air stripping of sulphur dioxide, SO2, from a sulphuric acid stream. An example in carbon capture and storage processes is the absorption of carbon dioxide, CO2, from gas streams such as atmospheric air and particularly from flue gases produced by carbonaceous fuel burning power generation plants into a solution having preferential absorption for CO2 compared to other gaseous components such as an aqueous solution of alkylamines. A second example in carbon capture and storage processes is desorption of CO2 from said absorbing solution after changes in operating conditions such as temperature and pressure. The efficacy of absorption or desorption is directly related to the uniformity of the liquid distribution.
A distributor may be considered as a single apparatus that may include several distribution stages such as a single inlet source of liquid that is first split into several but generally a few flows (for example, less than, but not necessarily limited to, 10) for a header or manifold system. Liquid is then distributed to a secondary system of several conduits, typically a greater number of conduits than in the first manifold, through one or more feed points in each secondary conduit. Each secondary conduit distributes liquid to many discharge points (e.g. >20); and may include a final stage of discharge means, such as down comer tubes, that direct the many discharge flows on to the packing. Additional stages of increasingly finer distribution can be contemplated, but preferable designs will limit these stages to as few as possible for cost-effectiveness.
There are many design variations for liquid distributors, but there are three distributor types generally recognized as pan or tray, closed conduit or pipe, and trough types. The pan or tray type of distributor has various means such as holes for a uniform liquid distribution but must also provide means such as gas risers for gas flow. The tray or pan type is seldom employed in towers larger than 1.5 meters diameter as they are relatively expensive and generally limited to smaller gas flows.
Pipe distributors are of relatively simple fabrication, generally using readily available piping components. A pipe distributor is typically an inlet pipe through the vessel side wall or vessel top head leading to a central manifold with several radial, horizontal pipe branches; or an inlet pipe into a single central horizontal pipe header through the wall and several perpendicular, horizontal side pipe branches; with a multitude of discharge orifices along the branch pipes. Pipe distributors can occupy a small overall cross-sectional area when designed for pressurized operation with high allowable pressure drop across small discharge orifices. However, disadvantages of pressurized pipe distributors include difficulty obtaining even liquid distribution when the inlet liquid also contains some gas or solids; requiring disassembly for cleaning; and producing fine liquid drops which are carried over with upward-flowing, high velocity gas.
Trough distributors use one or more, troughs to distribute the liquid throughout the tower. The troughs are generally arranged parallel to each other across the tower. The liquid distribution rate out of the troughs is controlled by the number of exit liquid discharge points, the size of the liquid discharge exits, and the surface height above the exits. An initial feed system comprised of a central feed pipe or feed trough is usually fed by means of an inlet pipe through the wall of the column, where the inlet pipe leads to the center of the feed pipe or feed trough or one end of the feed conduit. The initial feed system will split the inlet feed liquid into smaller flows to the distribution troughs and can be located above and perpendicularly across the lower troughs with liquid flow into each lower trough through a single inlet, or through two liquid flows from the opposite sides of the central feed pipe or trough, or through multiple liquid flows supplied by branches from the central feed pipe or feed trough. The trough type of distributor has an advantage over closed conduit type distributors of being open for easy inspection and solids clean out.
There are two main types of trough distributors based upon the kind of liquid exits: weir-type and orifice-type. Weir-type distributors have overflow weirs at or near the top of the trough, and are very sensitive to even small variations in liquid height having a large detrimental impact on uniform distribution. Orifice based distributors have submerged exits in the trough. Submerged orifices have flow rates less sensitive to the height of the liquid above them. However, orifices are more prone to becoming blocked with suspended solids that settle out when compared to weir-type distributors. Both orifices and weirs can be obstructed by large particles.
Distributors may also employ down comers, which are closed conduits, i.e. tubes, which further distribute liquid from discharge points of trough or conduit type distributors across the cross-section of the tower and down to the packing. These are effective in allowing for reduced number of distributor conduits while minimizing liquid entrainment within the gas stream.
In the sulphuric acid industry, pipe and trough distributors were traditionally made from ductile iron because of its ability to form a protective barrier to strong sulphuric acid. However, this barrier can be eroded off if the flow becomes turbulent. This means that the acid has to enter the distribution trough at a low velocity, which is generally achieved by having an overhead piping network to introduce the acid to the trough, splitting the total flow into smaller flows, at several points. Ductile iron troughs or pipes were also designed with large corrosion allowances making them very heavy.
Liquid introduced into packed towers will entrain solids, generally fine particles, from the slow wear of packing and other materials. Larger particles of solids found in the liquid are often small pieces of broken packing; usually occurring during the filling of the tower with the packing. Although means such as strainers or filters are employed to remove solids, such devices are not perfect and, in the sulphuric acid industry, the materials of construction suitable for filter elements have limited life. The solids in the liquid can build up deposits in distributors that cause mal-distribution and a periodic cleaning the equipment is required with subsequent loss of production. However, a higher liquid velocity will retard the formation of deposits by maintaining most solids in suspension to be swept out of the distributor.
Many distributors in sulphuric acid towers are now manufactured out of improved acid resistant materials allowing higher velocities in acid contacted equipment, piping, etc. with reduced size, weight, and corrosion. Cost-effective acid resistant metal alloys are austenitic stainless steels having high silicon content such as SARAMET®, registered to Aker Solutions Canada, Inc. for use in sulphuric acid plants. However, as liquid capacity is increased through a trough distributor, i.e. reducing size with higher velocities, difficulty arises in maintaining a calm liquid surface at a uniform height above each discharge exit; thus different methods of introducing the liquid into the troughs at multiple entrance points have been employed in order to maintain low velocity and minimal disturbance of the liquid surface. In large towers of diameters greater than about 2 meters, several feed conduits are typically employed to provide several liquid entry points into the distribution troughs. However, the additional feed conduits reduce cost-effectiveness and are inconvenient when cleaning is required.
There is, however, a need for an improved distributor, assembly and towers comprising such distributors.